1. Field of Invention
This invention relates to a vibratory transducer for detecting change in the frequency of vibration of a vibratory beam formed of a single silicon crystal due to a change in force of environment while vibrating the vibratory beam at its natural frequency.
2. Description of the Prior Art
There is known a transducer which comprises vibratory beams formed of a semiconductor, such as disclosed in UK Patent Application SN 24,186 filed in 1978, and corresponding to Japan Laid-Open Patent Publication No. 55-26,487, and described in FIGS. 1,2,3. FIG. 1 is a perspective view of the conventional transducer and FIG. 2 is a plan view of the transducer.
In FIGS. 1,2,3, a silicon substrate 1 is doped locally with boron at a density of 4.times.10.sup.19 atoms/cm.sup.3 more in certain regions (two regions are left as projections 12 and two regions are left as vibratory beams 14), and is masked at portion which is left as a frame 10. Silicon substrate 1 is then anisotropically etched in an aqueous solution of catechol and diamine acid to form frame 10, projections 12, partition 13 and vibratory beams 14. Vibratory beams 14 are disposed on top of projections 12.
FIG. 3 shows a transducer portion in fragmentary cross section. Vibratory beams 14 are disposed in spaced relation to the bottom 21 of the transducer, on which there are disposed a transducer driver electrode 22, a pickup electrode 23, and a protective electrode 24 Where the transducer is used as a pressure sensor, an end or frame thereof is attached to a pressure source to be measured.
Since a p.sup.+ layer is formed in the silicon substrate 1 to a depth of 1 micrometer by impurities of boron at a density of 4.times.10.sup.19 atoms/cm.sup.3, the conventional vibratory transducer has the following disadvantages.
(1) Due to different diameters of the atoms of silicon and boron, a slip line is created by lattice defects, resulting in deterioration of mechanical and electrical properties of the crystal. Consequently, the transducer is subjected to large residual stresses and has a varied temperature coefficient. It is thus difficult to produce a transducer which is highly sensitive and reliable.
(2) Since the density of the impurities is of a limited value that can be injected, the following deficiencies occur
(a) A pn-junction diode or transistor for detecting distortion cannot be formed in the transducer. PA1 (b) A MOS-FET of a buffer cannot be formed in the transducer even if a piezoelectric film for vibrating the vibratory beams is electrodeposited. PA1 (c) A piezoelectric resistor for detecting distortion of the vibratory beams cannot be formed in the transducer.